A device for measuring a rotation angle, also known as an angular decoder or rotation scanner, can be used to measure the rotation angle of rotatable object having a rotation shaft using the angular position of a mirror that is connected to said rotation shaft. Such a scanner can be widely used. A well known example, consists of a device wherein the mirror is connected to the shaft of a rotation motor, which motor and mirror together form a so-called galvanometer scanner. In manufacturing industry galvanometer scanners can be used for material processing by means of movable laser beams, such as welding, engraving or cutting work pieces. A more recent application is three-dimensional (3D-) prototyping, or printing. In optics, for example confocal microscopy galvanometer scanners are used for controlling X- and Y scanning of laser beams. Galvanometer scanners can also be used in apparatuses for medical treatments of very different types, such as cornea surgery, skin surgery and angioplasty. Also in a so-called fundus camera for scanning a human retina in an X- and Y direction a galvanometer scanner can be used.
In these and other applications the required resolution of the angular decoder is of the order of micro radians. A further requirement is that the scanning mirror can be moved rapidly over larger angles, for example 20 degrees in 1 millisecond. Therefore, the mass of that part of the angular decoder that is coupled to the rotation shaft should be small so that this part ads little to the moment of inertia of the scanner.
The requirement of high resolution and small moment of inertia leads to use of optical metrology in the angular decoder device, thus to an optical angular encoder device. A generally applied principle for measuring movements, including rotation, of objects is based on the use of optical gratings. For example U.S. Pat. No. 5,159,192 discloses a device for measuring the angular position of a galvanometer mirror which device uses a grating that is perpendicularly mounted on the rotating shaft of the mirror. The grating is reflective and ring-shaped and has radial grating lines. A light beam from a diode laser is divided into two sub-beams, which are directed to one and the same area of the grating. The sub-beams reflected by the grating are incident on a mirror, which reflects these sub-beams back towards the grating. Each of the twice-reflected sub-beams is captured by a separate radiation-sensitive detector. The two detector output signals provide information about the angle and direction of rotation of the shaft. To allow measuring angular positions with a resolution of the order of one micro radian the radius of the ring-shaped grating of this device should be of the order of centimeters. Since the moment of inertia of the grating disc is proportional to at least the fourth power of its radius, the required radius would set a limit to the galvanometer dynamics and velocity of the galvanometer mirror. Moreover, the measurement result of the device of U.S. Pat. No. 5,159,192 is sensitive to transverse shifts of the rotating shaft, because such shifts cause transverse displacements of the grating structure in a plane perpendicular the shaft axis and such displacement will change the pitch of the grating area observed by the optical elements of the device.
EP 0 651 232 shows another device wherein a reflective radial diffraction grating mounted on and rotating with a rotary object is used to measure the angular position of this object. This device uses a second, stationary grating to split a measuring beam from a source into two sub beams of different diffraction orders, which sub-beams are incident on different parts of the rotating grating. Sub-beams reflected by the rotating grating are brought to interference by the stationary grating in the plane of a comb type radiation-sensitive detector having a period corresponding to that of the interference pattern. The output signals of the detector provide information about the angular position of the rotary object. The device of EP 0 651 232 shows the same disadvantages as the device of U.S. Pat. No. 5,195,192.
DE Patent 10 2011 050 030 discloses another type of device for measuring the angular position of for instance a galvanometer mirror wherein a diffraction grating is used. In this device a converging beam of light is sent to a diffraction grating having a cylindrical surface that is centered on the rotation axis of the galvanometer via a mirror that is fixed to the rotation shaft of the mirror. The cylindrical grating reflects the beam back to the rotating mirror, which reflects light towards a lens that forms a moving image of this grating in the plane of a stationary grating that is arranged in front of a radiation-sensitive detector. The intensity of light on the detector varies periodically with the angular position of the mirror. For a cylinder radius r=15 mm and a grating period of 10 μm the detector signal will have a period of 0.67 milli radians. Since a convergent beam is used the rotation angle measurement is sensitive to transverse displacements of the rotation shaft. In practice it appears that such displacements of the order of a few micrometers cannot be avoided. A displacement Δx of the shaft in the direction perpendicular to the grating lines causes a measured angle error of Δx/r, wherein r is the radius of curvature of the cylindrical grating. If r=15 mm a displacement of the cylindrical grating over 1 μm will cause an error in angle measurement of 67 microradians, which is unacceptable in view of the measurement resolution now required. Moreover, because of its optical outlay and the number of optical elements, the device of DE Patent 10 2011 050 030 cannot be made compact and lightweight.